The present invention relates to shock absorbers that utilize a fluid pressure.
In general, cylinder-type hydraulic shock absorbers attached to suspension systems of automobiles or other vehicles are structured as follows. A piston connected with a piston rod is slidably provided in a cylinder having a hydraulic fluid sealed therein. The piston and the piston rod constitute in combination a piston assembly that is provided with a damping force generating mechanism including an orifice and a disk valve. The damping force generating mechanism generates a damping force by controlling, through the orifice and the disk valve, the flow of hydraulic fluid induced by sliding movement of the piston in the cylinder, which is caused by the extension and contraction of the piston rod.
When the piston speed is low (i.e. in a low piston speed region), the orifice generates a damping force of orifice characteristics (in which the damping force is approximately proportional to the square of the piston speed). When the piston speed is intermediate (i.e. in an intermediate piston speed region), the disk valve deflects to open according to the piston speed, thereby generating a damping force of valve characteristics (in which the damping force is approximately proportional to the piston speed). When the piston speed is high (i.e. in a high piston speed region), the amount of deflection of the disk valve increases up to a certain critical piston speed; therefore, the valve characteristics are maintained. When the critical piston speed is exceeded, the disk valve no longer deflects, and the flow path area is kept constant. Therefore, the orifice characteristics again become dominant. The critical piston speed can be adjusted by varying the cross-sectional area (flow path area) of a passage provided in the piston (piston passage), or by limiting the maximum amount of deflection of the disk valve.
In FIG. 5, the broken line shows the damping force characteristics obtained by a hydraulic shock absorber according to the related art. The related art apparatus enables damping force characteristics to be set for each of the low, intermediate and high piston speed regions. For the low piston speed region, damping force characteristics are set on the basis of the orifice area. For the intermediate piston speed region, damping force characteristics are set on the basis of the flexural rigidity of the disk valve when and after it has opened. For the high piston speed region, damping force characteristics are set on the basis of the flexural rigidity of the disk valve after it has opened, or based on the cross-sectional area (flow path area) of a passage provided in the piston.
One example of the related art is disclosed in Japanese Patent Application Publication No. Hei 2-195039. According to the related art technique, the disk valve is allowed to open stepwise by devising the shape of the disk valve seat and the configuration of disks constituting the disk valve, thereby suppressing a sharp change in damping force.
It is desirable that hydraulic shock absorbers be capable of reducing sharp changes in damping force characteristics and enable damping force characteristics to be set relatively easily.